We have an old rear end from a truck. Part of the rear end is simply buried in the ground, and there are supports coming off the above-ground part to keep it level. Ours is 45' and like TFX says, has upper support due to the size. Almost half the axle is buried in the ground.

I will say this. When used properly, it is the BEST tool for working a dog, if the dog will give the proper effort. But it is also a tool that you will need to stay on top of with a lot of dogs. Dogs WILL hurt themselves on a jenny, though I don't believe in them throwing out shoulders. Unlike a mill, they can, and will, jump after whatever bait you're using, and that in turn, can have them landing in awkward spots/positions.